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| United States Patent |
5,006,915
|
|
Yoshikawa
, et al.
|
April 9, 1991
|
Electric device and photoelectric conversion device comprising the same
Abstract
An electric device comprises a portion comprising an electrode, an n-type
inorganic semiconductor layer formed on the electrode, and an
electron-conductive organic layer formed on the n-type inorganic
semiconductor layer, and a photoelectric conversion device utilizes the
above electric device by using the electron-conductive organic layer as an
electron acceptor, with addition of an electron donor layer and another
electrode thereto.
| Inventors:
|
Yoshikawa; Masao (Yokohama, JP);
Suzuki; Tetsuro (Yokohama, JP)
|
| Assignee:
|
Ricoh Company, Ltd. (Tokyo, JP)
|
| Appl. No.:
|
477484 |
| Filed:
|
February 9, 1990 |
Foreign Application Priority Data
| Feb 14, 1989[JP] | 1-32798 |
| Jun 06, 1989[JP] | 1-142081 |
| Current U.S. Class: |
257/40; 136/263; 257/184; 257/E29.143; 257/E31.126; 257/E51.015 |
| Intern'l Class: |
H01L 029/28; H01L 023/54 |
| Field of Search: |
357/8,4,17,30,61
350/333,340,336
|
References Cited
U.S. Patent Documents
| 3575889 | Apr., 1971 | Klopffer et al. | 357/8.
|
| 3634336 | Jan., 1972 | Perez-Albueme | 357/8.
|
| 4127738 | Nov., 1978 | Ghosh et al. | 357/8.
|
| 4873556 | Oct., 1989 | Hyodo et al. | 357/8.
|
| 4907043 | Mar., 1990 | Uekita et al. | 357/8.
|
| Foreign Patent Documents |
| 0252756 | Jan., 1988 | EP | 357/8.
|
| 0144372 | Jun., 1987 | JP | 357/8.
|
| 7609163 | Feb., 1977 | NL | 357/8.
|
Primary Examiner: Hille; Rolf
Assistant Examiner: Clark; S. V.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Claims
What is claimed is:
1. A photoelectric conversion device comprising:
(a) a first electrode,
(b) an n-type inorganic semiconductor layer formed on said electrode,
(c) an electron-conductive organic layer formed on said n-type inorganic
semiconductor layer, which serves as an electron acceptor layer,
(d) an electron donor layer, and
(e) a second electrode, which are successively overlaid, and at least one
electrode of said first electrode or said second electrode is transparent
to employed light.
2. The photoelectric conversion device as claimed in claim 1, wherein at
least one electrode of said first electrode or said second electrode is
made of a metal having a work function of 4.5 eV or more.
3. The photoelectric conversion device as claimed in claim 1, wherein at
least one electrode of said first electrode or said second electrode is
transparent to employed light and has a thickness of 100 .ANG. to 10,000
.ANG..
4. The photoelectric conversion device as claimed in claim 1, wherein said
electron-conductive organic layer has a thickness of 100 .ANG. to 3,000
.ANG..
5. The photoelectric conversion device as claimed in claim 1, wherein said
first electrode is transparent for projection of light, and the
electron-conductive organic layer has a thickness of 100 .ANG. to 1,000
.ANG..
6. The photoelectric conversion device as claimed in claim 1, wherein said
electron donor layer comprises a phthalocyanine pigment.
7. The photoelectric conversion device as claimed in claim 1, wherein said
electron donor layer comprises an indigo pigment.
8. The photoelectric conversion device as claimed in claim 1, wherein said
electron donor layer comprises a thioindigo pigment.
9. The photoelectric conversion device as claimed in claim 1, wherein said
electron donor layer comprises a merocyanine pigment.
10. The photoelectric conversion device as claimed in claim 1, wherein said
electron donor layer has a thickness of 100 .ANG. to 3,000 .ANG..
11. The photoelectric conversion device as claimed in claim 1, wherein said
second electrode is transparent for projection of light, and the
electron-conductive organic layer has a thickness of 100 .ANG. to 1,000
.ANG..
12. The photoelectric conversion device as claimed in claim 1, wherein at
least one electrode of said first electrode or said second electrode is
transparent to employed light and has a thickness of 100 .ANG. to 10,000
.ANG., and the other electrode has a thickness of 50 .ANG. to 3,000 .ANG.
and a work function of 4.5 eV or more.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electric device using organic materials, which
is applicable to a solar battery, a photoelectric conversion device and a
transistor, and more particularly to an electric device comprising a
portion in which an electrode, an n-type inorganic semiconductor layer,
and an electron-conductive organic layer are successively overlaid in this
order.
2. Discussion of Background
Electric devices using inorganic materials have conventionally been used
for solar batteries, photoelectric conversion devices and transistors. In
recent years, however, electric devices using organic materials have taken
over the place of the conventional electric devices. This has been made
possible by the recent improvement and diversification in characteristics
of organic semiconductors, and the progress of thin-film forming
technique.
It is preferable that such electric devices have an electric current
density of 1 mA/cm.sup.2 or more. In the case of a solar battery, for
instance, a short-circuit photoelectric current density of 10 mA/cm.sup.2
or more is necessary in order to attain a conversion efficiency of 5% or
more under sunlight. An organic electroluminescence device is also
required to have an electric current density of approximately 10
mA/cm.sup.2 so as to obtain a luminance as high as 1000 cd/m.sup.2.
Furthermore, a transistor capable of flowing a great amount of electric
current can be utilized for various purposes applications.
As described above, it is an indispensable requirement that the electric
device have a high electric current density from the viewpoint of
practical use.
In order to attain a high electric current density, it is necessary that
both positive holes and electrons can freely move inside the electric
device. However, there are few organic materials which can fulfill this
requirement. In order to satisfy the requirement, it has been proposed
that two organic layers, one serving as an electron-conductive layer, and
the other serving as a positive hole-conductive layer, be separately
formed.
However, even when the electric device comprising the above two layers, the
electric current density decreases in the case where an energy barrier
exists therein. The energy barrier is formed at a junction of any two
layers made of different materials. Therefore, it is necessary that all
junctions except those in a function site be in an ohmic contact which
does not hinder the migration of electric charges. In the case of a
photoelectric conversion device such as a solar battery, a site in which
photoelectric charges are generated is the function site.
The inventors of the present invention have carefully studied the above
proposal, in which two organic layers serving as an electron-conductive
layer and as a positive hole-conductive layer are separately formed in
order to attain a high electric current density. As a result, problems
were found as described below in the materials for a negative electrode.
In general, metals have conventionally been used as the materials for a
negative electrode. In particular, metals having a work function of less
than 4.5 eV have been used so that the negative electrode can come into
ohmic contact with an organic semiconductor layer.
For example, a photoelectric conversion device which has a maximum
short-circuit electric current density of 3 mA/cm.sup.2 is disclosed in
Japanese Patent Publication 62-4871. In this device, Ag and In are used as
the materials for the negative electrode. It is generally known that these
metals are readily oxidized.
With respect to an electroluminescence device, a device using polyvinyl
carbazole as a luminescent material has been reported by R. H. Partridge
in Polymer, 24, 748 (1983). In this device, Cs is used as the material for
the negative electrode.
Furthermore, in Appl. Phys. Lett., 51, 913 (1987), C. W. Tang has reported
an electric device composed of an aluminum quinoline complex layer and a
diamine layer. In this device, the negative electrode is prepared by using
an alloy of Mg and Ag which is also easily oxidized.
It is considered when metals having a work function of 4.5 eV or more are
used as the materials for a negative electrode, the stability of the
electrode can be enhanced, and the finished device can withstand practical
use.
However, when such metals are used as they are for the electrode, the
electric current density of the device is drastically decreased, and the
characteristics of the device are deteriorated. This is because a barrier
to electron conduction is formed between an electron-conductive organic
semiconductor layer and an electrode made of a metal having a work
function of 4.5 eV or more.
A photoelectric conversion device is a device which converts light applied
thereto to an electric energy. In general, a photoelectric conversion
device using organic materials has a lower conversion efficiency than a
device using inorganic materials. This is because organic materials have a
low short-circuit electric current density (J.sub.sc) and a small fill
factor (ff).
A device having a conversion efficiency of 5% is required to have a
short-circuit electric current density J.sub.sc) of at least 10
mA/cm.sup.2 when a white light with an intensity of 75 mW/cm.sup.2 is
applied thereto. However, as will be described later, a known solar
battery using organic materials has a short-circuit electric current
density which is much lower than the above value, and has an insufficient
value of the fill fuctor (ff). One of the reasons for the low fill fuctor
is considered that the quantum efficiency of organic materials drastically
decreases in a weak electric field. In order to overcome this problem, the
device is required to generate a strong inner electric field. Furthermore,
in order to obtain a large fill factor, it is also required that electric
charges generated can reach the electrode without being hindered by an
electrical barrier.
Conventional electric devices are classified into three types as described
below depending on the fabrication method thereof.
(1) Electric devices containing a Schottky junction or a metal insulator
semiconductor (MIS) junction:
These devices utilize an inner electric field generated at a junction of a
metal and a semiconductor layer. Organic materials such as merocyanine
dyes and phthalocyanine pigments have been reported as the materials for
the semiconductor layer for use in the device of this type.
An electric device composed of Al, a merocyanine dye and Ag shows a
conversion efficiency of 0.7% when a white light with an intensity of 78
mW/cm.sup.2 is applied thereto, as reported by A. K. Ghosh, et al. in J.
Appl. Phys., 49, 5982 (1978). Other data regarding this device are as
follows:
Open-circuit electromotive force (V.sub.oc)=1.2 V,
Short-circuit current density (J.sub.sc)=1.8 mA/cm.sup.2, and
Fill factor (ff)=0.25.
Only a p-type organic semiconductor can show a high conversion efficiency,
so that metals having a low work function of less than 4.5 eV, such as Al,
In and Mg, are employed. These metals are readily oxidized to form an
MIS-type junction.
The open-circuit electromotive force (V.sub.oc) of the device of this type
is relatively high. However, since metals having a work function of less
than 4.5 eV, such as Al, In and Mg, are used as the materials for the
electrode, the electrode has the disadvantages of low transparency and low
stability due to oxidization. Practically, the transparency of the
electrode is, in general, approximately 10%, and at most approximately
30%. Thus, it cannot be expected that the device of this type can have a
high conversion efficiency
(2) Electric devices containing a hetero p/n junction of an n-type
inorganic semiconductor layer and a p-type organic semiconductor layer:
These devices utilize an electric field which is generated when the n-type
and p-type semiconductor layers are in contact.
CdS and ZnO are used as the materials for the n-type inorganic
semiconductor. Merocyanine dyes and phthalocyanine pigments (Pc) have been
reported as the materials for the p-type organic semiconductor.
An electric device composed of ITO, CdS electrically deposited onto the ITO
layer, CACPc and Au shows a conversion efficiency of 0.22% when an AM-2
light with an intensity of 75 mW/cm.sup.2 is applied thereto, as reported
by A. Horr, et al. in Appl. Phys. Lett, 42, 15 (1983). Other data
regarding this device are as follows: V.sub.oc =0.69 V, J.sub.sc =0.89
mA/cm.sup.2, and ff=0.29.
An electric device composed of ITO, ZnO, a merocyanine dye and Ag shows a
conversion efficiency of approximately 0.5% when a white light with an
intensity of 70 mW/cm.sup.2 is applied thereto, as reported by K. Kudo, et
al. in J. Appl. Phys., 19, L683 (1980). Other data regarding this device
are as follows: V.sub.oc =0.4 V, J.sub.sc =1.1 mA/cm.sup.2, and
ff=0.3-0.4.
In the device of this type, electric charges are mainly generated in the
organic layer which is generally made of a single organic material, so
that the generation of electric charges is affected by the spectral
sensitivity of the organic layer. No organic semiconductors have been
found until now which are capable of strongly absorbing light in a wide
wavelength range of 400 nm to 800 nm.
Therefore, the device of this type can overcome the shortcomings of low
transparency and low stability of the device of the above type (1).
However, the device is restricted by the spectral sensitivity of the
organic material employed, and cannot show a high conversion efficiency.
(3) Electric devices containing a hetero junction of organic layers:
The devices utilize an electric field which is generated when (i) an
organic material of an n-type or having high electron attractive ability
and (ii) a p-type organic material are in contact.
Dyes such as Malachite Green, Methyl Violet and a pyrylium compound, and
condensed polycyclic compounds such as flavanthrone and perylene pigments
have been reported as specific examples of the former organic material.
As the latter organic material, phthalocyanine (Pc) pigments and
merocyanine dyes have been reported.
An electric device composed of ITO, CuPc, a perylene pigment and Ag shows a
conversion efficiency of 0.95% when an AM-2 light with an intensity of 75
mW/cm.sup.2 is applied thereto, as reported by C. Tang in Appl. Phys.
Lett, 48, 183 (1986). Other data regarding this device are as follows
V.sub.oc =0.45 V, J.sub.sc =2.3 mA/cm.sup.2, and ff=0.65.
With respect to the electric devices containing organic materials, the
above value of the conversion efficiency is maximum so far. C. Tang also
discloses, in Japanese Patent Publication 62-4871, a device having the
same structure as that of the above device but using a perylene pigment of
a different type. The conversion efficiency of this device is 1% (V.sub.oc
=0.44 V, J.sub.sc =3.0 mA/cm.sup.2, ff=0.6).
The device of this type is most preferable among the devices described
above since light can be applied to the transparent electrodes. Moreover,
photoelectric charges can be generated in two organic layers, so that the
range of spectral sensitivity can be broadened.
In fact, in view of the spectral sensitivities reported by Tang in the
Appl. Phys. Lett., it is considered that electric charges are generated in
the perylene pigment layer when light with a wavelength of 450 to 550 nm
is applied to the device, but in the CuPc layer, electric charges are
generated when light of 550 to 700 nm is applied. The device according to
Tang shows a higher fill fuctor (ff) than those of other devices. From
this fact it is considered that in the Tang's device, a stronger inner
electric field is generated than those generated in other devices. The
device according to Tang, however, still has the shortcomings described
below.
(i) Since the organic layer is as thin as 300 to 500 .ANG., (which
thickness is particularly described as a preferable thickness in Tang's
article), it is apt to be suffered from a problem of pin holes. According
to the experiment carried out by the inventors of the present invention,
the probability of the occurrence of the short-circuit between two
electrodes due to pin holes is relatively high.
The area of the electrode of the Tang's device is only 0.1 cm.sup.2 when an
area of 1 cm.sup.2 or more is necessary for practical use. Thus the yield
of the production of a larger electrode will become a problem in practical
use.
(ii) According to Tang, it is required that the electrode be in ohmic
contact with each organic layer. He has reported that the value of
V.sub.oc decreases when the device is composed of ITO, a perylene pigment,
CuPc and Ag which are overlaid in this order. It is considered that this
is because the ITO layer is not in ohmic contact with the perylene pigment
layer, and the CuPc layer is also out of ohmic contact with the Ag layer.
In order to attain this ohmic contact, a metal which is in contact with an
electron-accepting organic material is required to be stable. Such a metal
has a low work function, and In, Ag, Sn and A( are disclosed as such
metals. However, these metals are readily oxidized.
SUMMARY OF THE INVENTION
Accordingly, a first object of the present invention is to provide an
electric device comprising (i) an electron-conductive organic layer and
(ii) an electrode made of a stable metal, which is capable of stably
flowing a large amount of electric current without decreasing the current
density.
A second object of the present invention is to provide a photoelectric
conversion device using the above-described electric device.
The first object of the present invention can be attained by an electric
device comprising a portion which comprises an electrode, an n-type
inorganic semiconductor layer formed on the electrode, and an
electron-conductive organic layer formed on the n-type inorganic
semiconductor layer.
The second object of the present invention can be attained by a
photoelectric conversion device comprising (a) a first electrode, (b) an
n-type inorganic semiconductor layer formed on the first electrode, (c) an
electron-conductive organic layer formed on the n-type inorganic
semiconductor layer, which serves as an electron acceptor layer, (d) an
electron donor layer, and (e) a second electrode, which are successively
overlaid, and at least one electrode of the first electrode or the second
electrode is transparent to employed light.
BRIEF DESCRIPTION OF THE DRAWING
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood with reference to the following detailed description when
considered in connection with the accompanying drawing, wherein:
the sole FIGURE is a cross-sectional view of an example of a photoelectric
conversion device according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the present invention, a large amount of electric current can
be obtained by an electric device which includes a portion comprising an
electrode made of a metal having a work function of 4.5 eV or more, on
which an n-type inorganic semiconductor layer and an electron-conductive
organic layer are successively overlaid.
When a metal having a work function of 4.5 or more is brought into close
contact with an electron-conductive organic layer, a Schottky junction is
formed. This junction serves as an energy barrier to electrons which
migrate from the electrode to the electron-conductive organic layer and
vice versa.
In the case where an n-type inorganic semiconductor layer is interposed
between the electrode and the electron-conductive organic layer, the bulk
current capacity of the n-type inorganic semiconductor layer, and the
junction current capacity between the n-type inorganic semiconductor layer
and the electrode become larger than the bulk current capacity of the
electron-conductive organic layer. In addition, the interface between the
n-type inorganic layer and the electron-conductive organic layer becomes
an ohmic junction, so that the current capacity of the whole device is not
lowered.
In the present invention, the following materials are preferably employed
for the electrode, the n-type inorganic semiconductor layer and the
electron-conductive organic semiconductor layer.
Electrode: Metals having a work function of 4.5 eV or more, such as Au, Pd,
Pt, Cr, Ni and C.
n-type inorganic semiconductor layer: n-type crystalline silicon, n-type
amorphous silicon, phosphorousdoped amorphous silicon, ZnS, ZnO,
trivalent-metal-doped ZnO, ZnSe, CdS, CdSe, and TiO. Of these, ZnO and CdS
are more preferable for use in the present invention.
Electron-conductive organic semiconductor layer: electron acceptor
compounds such as tetracyanoquinodimethane, tetracyanoethylene,
dimethyldichlorobenzoquinone and trinitrofluorenone; perylene pigments
such as Pigment Red (hereinafter referred to as "PR") 179, PR 190, PR 149,
PR 13, PR 189 and Pigment Orange 26; perynone pigments such as Pigment
Orange 43 and PR 194; quinone pigments such as PR 168, PR 177, PY 24 and
Vat Yellow 4; conjugated polymeric materials such as polyacetylenes and
polythiophenes, doped with K or Na; quinone-containing yellow pigments
such as flavanthrone; and dyes such as Crystal Violet, Methyl Violet and
Malachite Green.
In the present invention, the above-described electrode, the n-type
inorganic semiconductor layer and the electron-conductive organic
semiconductor layer are successively overlaid in this order. Other layers
or a positive electrode layer may be overlaid on the electron-conductive
organic semiconductor layer, if necessary.
For example, in the case of a photoelectric conversion device, an organic
coloring layer in which positive holes can move, and a positive electrode
layer are further provided, so that photoelectric conversion is conducted
at the interface between two organic layers. In the case of an
electroluminescence device, a luminescent layer is further provided.
EXAMPLE 1-1
ZnO was deposited onto an ITO-coated surface of a glass substrate by the DC
magnetron sputtering method, the substrate heated to a temperature of
approximately 300.degree. C., using an argon gas as the carrier gas for
the sputtering. Thus, a ZnO layer, that is, an n-type inorganic
semiconductor layer, with a thickness of approximately 1300 .ANG., was
formed on the ITO electrode.
Perylene tetracarboxylic acid methylimide was deposited onto the ZnO layer
by vacuum deposition, whereby an electron-conductive organic layer having
a thickness of approximately 2000 .ANG. was formed on the ZnO layer.
Gold was then deposited onto the above-formed organic layer by vacuum
deposition so as to form a positive electrode. The facing area between the
ITO electrode and the positive electrode was made 0.25 cm.sup.2.
A lead line was attached to the two electrodes by using a silver paste.
Thus, electric device No. 1-1 according to the present invention was
obtained.
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -1.3.times.10.sup.-3 A/cm.sup.2.
COMPARATIVE EXAMPLE 1-1
The procedure for Example 1-1 was repeated except that the ZnO layer formed
as the n-type inorganic semiconductor layer in Example 1-1 was eliminated,
whereby comparative electric device No. 1-1 was obtained.
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -7.times.10.sup.-5 A/cm.sup.2.
EXAMPLE 1-2
The procedure for Example 1-1 was repeated except that the thickness of the
perylene pigment layer formed in Example 1-1 as the electron-conductive
organic layer was changed from about 2000 .ANG. to 800 .ANG., whereby
electric device No. 1-2 according to the present invention was obtained.
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -2.6.times.10.sup.-3 A/cm.sup.2.
EXAMPLE 1-3
The procedure for Example 1-1 was repeated except that the perylene
tetracarboxylic acid methylimide used in Example 1-1 to form the
electron-conductive organic layer was replaced by a compound having the
following formula, whereby electric device No. 1-3 according to the
present invention was obtained.
##STR1##
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -2.2.times.10.sup.-3 A/cm.sup.2.
EXAMPLE 1-4
The procedure for Example 1-1 was repeated except that the perylene
tetracarboxylic acid methylimide used in Example 1-1 to form the
electron-conductive organic layer was replaced by a compound having the
following formula, and that the thickness thereof was changed from about
2000 .ANG. to 1300 .ANG., whereby electric device No. 1-4 according to the
present invention was obtained.
##STR2##
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -1.5.times.10.sup.-3 A/cm.sup.2.
EXAMPLE 1-5
The procedure for Example 1-1 was repeated except that the perylene
tetracarboxylic acid methylimide used in Example 1-1 to form the
electron-conductive organic layer was replaced by a compound having the
following formula, and that the thickness thereof was changed from about
2000 .ANG. to 1300 .ANG., whereby electric device No. 1-5 according to the
present invention was obtained.
##STR3##
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -3.2.times.10.sup.-3 A/cm.sup.2.
COMPARATIVE EXAMPLE 1-2
The procedure for Example 1-2 was repeated except that the ZnO layer formed
in Example 1-2 as the n-type inorganic semiconductor layer was eliminated,
whereby comparative electric device No. 1-2 was obtained.
The device thus obtained was found to be short-circuited.
COMPARATIVE EXAMPLE 1-3
The procedure for Example 1-3 was repeated except that. the ZnO layer
formed in Example 1-3 as the n-type inorganic semiconductor layer was
eliminated, whereby comparative electric device No. 1-3 was obtained.
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -1.3.times.10.sup.-4 A/cm.sup.2.
COMPARATIVE EXAMPLE 1-4
The procedure for Example 1-4 was repeated except that the ZnO layer formed
in Example 1-4 as the n-type inorganic semiconductor layer was eliminated,
whereby comparative electric device No. 1-4 was obtained.
A voltage with a sweep rate of 6 mV/s was applied to the electrode of the
device thus obtained, and the electric current flowing through the device
was measured at .+-.1 V. As a result, the current density at -1 V was
determined to be -2.3.times.10.sup.-5 A/cm.sup.2.
COMPARATIVE EXAMPLE 1-5
The procedure for Example 1-5 was repeated except that the ZnO layer formed
in Example 1-5 as the n-type inorganic semiconductor layer was eliminated,
whereby comparative electric device No. 1-5 was obtained.
A voltage with a sweep rate of 6 mV/s was applied to the ITO electrode of
the device thus obtained, and the electric current flowing through the
device was measured at .+-.1 V. As a result, the current density at -1 V
was determined to be -4.5.times.10.sup.-5 A/cm.sup.2.
In summary, the effects of the electric device according to the present
invention are as follows:
(a) The current density is maintained high.
(b) Even if the electron-conductive organic layer is thin, the probability
of the occurrence of short-circuit caused by pin holes can be minimized.
The photoelectric conversion device according to the present invention can
be prepared by using the abovedescribed electric device of the present
invention together with one or more electrodes. In this photoelectric
conversion device, at least one electrode is transparent to light to be
employed. Furthermore, the electron-conductive organic layer is deposited
with an electron donor type organic material so that it can serve as an
electron acceptor layer. Namely, an n-type inorganic semiconductor layer,
an electron-conductive organic layer which serves as an electron acceptor
layer, an electron donor organic layer, and a second electrode are
successively overlaid on a transparent first electrode in the
photoelectric conversion device of the present invention.
The reasons why the conversion efficiency can be improved and the
probability of the occurrence of the short-circuit can be reduced when the
n-type inorganic semiconductor layer is present are not clear but is
considered as follows:
Reasons for Improvement in Conversion Efficiency
(1) One electrode, the n-type inorganic semiconductor layer overlaid on the
electrode, and the electron acceptor organic layer are in an ohmic contact
with one another. Electrons among photoelectric charges generated at the
interface between the electron acceptor organic layer and the electron
donor organic layer can move freely without being hindered by a barrier.
(2) A junction is formed at the interface between the n-type inorganic
semiconductor layer and the electron acceptor organic layer. Owing to this
junction, positive holes generated upon application of light migrate to
the organic layer, and electrons migrate to the inorganic semiconductor
layer. Photoelectric charges are generated even at this interface.
(3) Electrons are supplied from the n-type inorganic semiconductor layer to
the electron acceptor organic layer in the dark, so that the electric
field generated at the interface between the electron acceptor organic
layer and the electron donor organic layer becomes strong.
Reasons for Reduction of Probability of Short-Circuit
(1) The difference in level at the edge of the first electrode can be
decreased when the n-type inorganic semiconductor layer exists.
(2) Even when pin holes are formed on the organic layer, no short-circuit
is occurred. This is because the n-type inorganic semiconductor layer has
resistance which is higher than that of the electrode.
Referring now to the accompanying drawing, the structure of the
photoelectric conversion device according to the present invention will be
explained.
On one surface of a transparent insulation substrate 1, a transparent first
electrode 2, a transparent n-type inorganic semiconductor layer 3, an
electron acceptor organic layer 4, an electron donor organic layer 5, and
a second electrode 6 are successively overlaid in this order. A lead line
7 is connected with both the first electrode 2 and the second electrode 6.
The substrate 1 can be positioned on the side of the second electrode 6. In
addition, the electron acceptor organic layer 4 and the electron donor
organic layer 5 can be reversely provided. In this case, it is not
necessary that the n-type inorganic semiconductor layer 3 be transparent;
and the layer 3 is placed on the side of the second electrode 6. However,
the most preferable structure is shown in the FIGURE.
The photoelectric conversion device fabricated in the above-described
structure shows a high conversion efficiency although it contains organic
materials.
The n-type inorganic semiconductor layer contributes to the improvement of
the conversion efficiency, that is, the improvement of V.sub.oc, J.sub.sc
and ff, and to the reduction of the probability of the occurrence of the
short-circuit. Furthermore, since the second electrode can be made of a
stable metal having a work function of 4.5 eV or more, the electrode is
free from oxidization.
The component layers of the photoelectric conversion device according to
the present invention will be explained below.
(a) Transparent insulation substrate
Glass and a synthetic plastic film which can withstand a temperature of
approximately 150.degree. C. are suitably used as materials for the
substrate.
(b) Transparent electrode
Indium tin oxide (ITO), tin oxide or indium oxide is used. Of these, ITO
and tin oxide are preferable since transparency and electron conductivity
are the essentially required properties. The thickness of the electrode is
preferably 100 to 10000 .ANG..
(c) n-type inorganic semiconductor layer
The thickness of this semiconductor layer is preferably 10 to 10000 .ANG..
(d) Electron acceptor organic layer
The materials for the electron-conductive organic layer mentioned
previously can be used for forming this layer.
This layer can be formed by vacuum deposition, spin coating or dipping. Of
these, vaccum deposition is preferable in order to form a thin and uniform
layer.
The thickness of the layer is preferably 100 to 3000 .ANG.. However, in the
case where the first electrode is transparent for projection of light, the
preferred thickness of this layer is 100 to 1000 .ANG..
(e) Electron donor organic layer
Phthalocyanine pigments containing Cu, Zn, Co, Ni, Pb, Pt, Fe, Mg, Al or Ti
as a center metal, phthalocyanine pigments containing no center metal,
indigo and thioindigo pigments such as Pigment Blue 66 and Pigment Violet
36, quinacridone pigments such as Pigment Violet 19 and Pigment Red 122,
and merocyanine dyes can be used for forming this layer.
This layer can be formed by vacuum deposition, spin coating or dipping. Of
these, vaccum deposition is preferable in order to form a thin and uniform
layer.
The thickness of the layer is preferably 100 to 3000 .ANG.. However, in the
case where the second electrode is transparent for projection of light,
the preferred thickness of this layer is 100 to 1000 .ANG..
(f) Second electrode
Metals having a work function of 4.5 eV or more, such as Au, Pt, Ni, Pd, Cu
and Cr, can be used. Of these, Au is most preferable since it is highly
stable.
The thickness of the electrode is preferably 50 to 3000 .ANG..
EXAMPLE 2-1
A glass substrate coated with ITO (ITO thickness: 900 .ANG.) made by
Matsuzaki Vacuum K.K. was thoroughly washed. ZnO was deposited onto the
surface of the ITO-coated surface of the glass substrate by the DC
magnetron sputtering method with the substrate heated to a temperature of
approximately 300.degree. C., using an argon gas as the carrier gas for
the sputtering. Thus, a ZnO layer, that is, an n-type inorganic
semiconductor layer, with a thickness of approximately 2800 .ANG. was
formed on the ITO first electrode.
Perylene tetracarboxylic acid methylimide was deposited onto the ZnO layer
by vacuum deposition at room temperature, whereby an electron acceptor
organic layer with a thickness of approximately 500 .ANG. was formed on
the ZnO layer. Thereafter, aluminum phthalocyanine chloride (ACPc) was
deposited onto the above-formed electron acceptor organic layer, thereby
forming an electron donor organic layer having a thickness of
approximately 400 .ANG..
Gold was then deposited onto the ACPc layer by vacuum deposition so as to
form a second electrode. The facing area between the first electrode and
the second electrode was made 0.25 cm.sup.2.
A lead line was attached to the above two electrodes by using a silver
paste. Thus, photoelectric conversion device No. 2-1 according to the
present invention was obtained.
The conversion efficiency of the device thus obtained was determined by
applying a pseudo sun light (AM-1.5) with an intensity of 75 mW/cm.sup.2
and a voltage with a sweep rate of 6 mV/s to the ITO electrode. As a
result, it was found that V.sub.oc was 0.47 V, ff was 0.45, and J.sub.sc
was 2.58 mA/cm.sup.2. From the above data, the conversion efficiency was
determined to be 0.74%.
COMPARATIVE EXAMPLE 2-1
The procedure for Example 2-1 was repeated except that the ZnO layer formed
in Example 2-1 as the n-type inorganic semiconductor layer was eliminated,
whereby photoelectric conversion device No. 2-1 according to the present
invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.26 V, ff
was 0.33 and J.sub.sc was 2.0 mA/cm.sup.2.From the above data, the
conversion efficiency was determined to be 0.23%. The values of V.sub.oc
scattered widely, and they were mostly in a range of 0.26 V or less.
EXAMPLE 2-2
The procedure for Example 2-1 was repeated except that the thickness of the
electron acceptor organic layer was changed from 500 .ANG. to 800 .ANG.,
and that aluminum phthalocyanine chloride used in Example 2-1 for forming
the electron donor organic layer was replaced by titanyl phthalocyanine
(TiOPc), and that the thickness of the TiOPc layer was changed from 400
.ANG. to 300 .ANG., whereby photoelectric conversion device No. 2-2
according to the present invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.5 V, ff
was 0.46 and J.sub.sc was 1.6 mA/cm.sup.2. From the above data, the
conversion efficiency was determined to be 0.5%.
COMPARATIVE EXAMPLE 2-2
The procedure for Example 2-2 was repeated except that the ZnO layer formed
in Example 2-2 as the n-type inorganic semiconductor layer was eliminated,
whereby comparative photoelectric conversion device No. 2-2 was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.44 V, ff
was 0.39 and J.sub.sc was 0.99 mA/cm.sup.2. From the above data, the
conversion efficiency was determined to be 0.22%.
EXAMPLE 2-3
The procedure for Example 2-1 was repeated except that the thickness of the
ZnO layer formed in Example 2-1 as the n-type inorganic semiconductor
layer was changed from 2800 .ANG. to 500 .ANG., whereby photoelectric
conversion device No. 2-3 according to the present invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.47 V, ff
was 0.45 and J.sub.sc was 2.46 mA/cm.sup.2. From the above data, the
conversion efficiency was determined to be 0.69%.
EXAMPLE 2-4
The procedure for Example 2-1 was repeated except that the thickness of the
ZnO layer formed in Example 2-1 as the n-type inorganic semiconductor
layer was changed from 2800 .ANG. to 6000 .ANG., whereby photoelectric
conversion device No. 2-4 according to the present invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.48 V, ff
was 0.43 and J.sub.sc was 2.55 mA/cm.sup.2. From the above data, the
conversion efficiency was determined to be 0.7%.
EXAMPLE 2-5
The procedure in Example 2-1 was repeated except that the argon gas used in
Example 2-1 at the time of the formation of the ZnO layer was replaced by
a 1:1 mixed gas of argon and oxygen, and the thickness of the ZnO layer
was changed from 2800 .ANG. to 1000 .ANG., whereby photoelectric
conversion device No. 2-5 according to the present invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.47 V, ff
was 0.45 and J.sub.sc was 2.2 mA/cm.sup.2. From the above data, the
conversion efficiency was determined to be 0.62%.
EXAMPLE 2-6
The procedure for Example 2-1 was repeated except that the argon gas used
in Example 2-1 at the time of the formation of the ZnO layer was replaced
by a 1:1 mixed gas of argon and oxygen, and the thickness of the ZnO layer
was changed from 2800 .ANG. to 600 .ANG., whereby photoelectric conversion
device No. 2-6 according to the present invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.47 V, ff
was 0.43 and J.sub.sc was 1.9 mA/cm.sup.2. From the above data obtained,
the conversion efficiency was determined to be 0.51%.
EXAMPLE 2-7
The procedure for Example 2-1 was repeated except that the ZnO used in
Example 2-1 for forming the n-type inorganic semiconductor layer was
replaced by ZnO doped with 2% of aluminum, whereby photoelectric
conversion device No. 2-7 according to the present invention was obtained.
The conversion efficiency of this device was determined in the same method
as in Example 2-1. As a result, it was found that V.sub.oc was 0.47 V, ff
was 0.41 and J.sub.sc was 2.7 mA/cm.sup.2. From the above data, the
conversion efficiency was determined to be 0.7%.
The above examples and comparative examples demonstrate that the
photoelectric conversion devices according to the: present invention show
a high conversion efficiency although they contain organic materials as
the electron acceptor and donor materials.
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